Nowadays, liquid foods are most generally packed and transported in packaging containers of single-use disposable type, and a very large group of these so-called single-use disposable packages is produced from a packaging laminate comprising a core layer of paper or paperboard and outer coatings of a thermoplastic, normally polyethylene. Well-known examples of such commercial single-use disposable packages are those sold under the registered trademarks Tetra Brik®, Tetra Rex® and Tetra Top®.
Packaging containers produced from a packaging laminate which exclusively consists of paper and/or paperboard layers and outer coatings of polyethylene are per se sufficiently mechanically strong and stable to withstand outer stresses in normal transport and handling, but almost totally lack the requisite tightness properties vis-á-vis gases for the reliable storage of oxygen gas-sensitive products such as juice and wine which very rapidly deteriorate in contact with oxygen gas. In order to impart to the packaging containers the requisite tightness properties against gases, in particular oxygen gas, the prior art packaging laminate is therefore supplemented with a so-called barrier layer between the paper or paperboard layer and the one outer coating.
Normally, use is made as barrier layer of an extremely thin aluminium foil (‘Alifoil’) which, in addition to its extremely good tightness properties, also enjoys the advantage that it makes for thermosealing by inductive heating of the packaging laminate when this is reformed into packaging containers. However, a problem is that an aluminium foil lacks ductility or extensibility and, as a result, shows a tendency to rupture or crack when it is subjected to extreme tensile stresses in connection with the reforming of the packaging laminate into packaging containers, whereby the desired tightness properties may readily deteriorate or be lost altogether. Moreover, an aluminium foil is extremely expensive and, as a result, contributes in unnecessarily increasing the material costs of the packaging laminate.
Another example of barrier layers in a packaging laminate of the above-described type is a layer of a so-called barrier polymer, and the most common of these barrier polymers is a copolymer of ethylene and vinyl alcohol (EVOH). An EVOH layer possesses barrier properties against gases, in particular oxygen gas, almost on a par with an aluminium foil, but in addition enjoys the major advantage that it is sufficiently ductile or extensible so as not to rupture or crack when it is subjected to extreme tensile stresses during the reforming of the packaging laminate into packaging containers. Furthermore, an EVOH layer is more economical than a comparable aluminium foil. On the other hand, an EVOH layer, like layers of other known barrier polymers such as, for example, polyvinyl alcohol and certain polyamides, is extremely moisture-sensitive and, as a result, rapidly loses its gas tightness properties when coming into contact with moisture or liquid. A certain protection against the action of moisture and liquid is imparted to the EVOH layer by both of the outer coatings of thermoplastic of the packaging laminate, but this protection is relatively short-lived and sufficient only during a limited, first period of time of the normal service life of a packaging container.
In an earlier attempt to solve the above-described problem with insufficient moisture and liquid protection, the prior art packaging laminate has been provided with thicker outer coatings of thermoplastic, preferably polyethylene. Granted, this solution imparts to the moisture-sensitive barrier layer an extended moisture and liquid protection, but a thicker outer coating of thermoplastic in direct contact with certain types of foods such as fruit juices gives the packed food poorer product protection because of an increased absorption of essential oils and aromatic substances (so-called scalping).
Another earlier attempt to impart to the prior art packaging laminate increased moisture and liquid protection has taken as its point of departure making the moisture- and liquid-sensitive barrier layer thicker, but this proposed solution entails increased material consumption and consequentially increased material costs for the packaging laminate.